split: remove Box<dyn Read>

[oech3]

This is needed to define .as_fd() to enable io::copy's specialization (e.g. reflink via copy_file_range).
Part of #13073 .

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will degrade performance by 84.15%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 6 untouched benchmarks
⏩ 378 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	split_number_chunks	132 µs	5,452.7 µs	-97.58%
⚡	Simulation	split_bytes	282.6 µs	272.4 µs	+3.75%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing oech3:split-boxr (ce1fcc2) with main (a60058c)}

Footnotes

1. 378 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩